254 A MONOGRAPH OF THE 



Retentive spicula biclentate and tridentate equi- 

 anchorate, abundant and frequently very large, dis- 

 persed. 



Colour. When dried, deep nut-brown. 

 Habitat. Strangford Lough, Professor Dickie. 

 Examined. In the dried state. 



I am indebted to Professor Dickie for a fragment of this 

 species, about three quarters of an inch in diameter, and 

 the eighth of an inch thick. 



This sponge differs greatly in external appearance from 

 Halichondria incrustans, although in the structure of the 

 skeleton and in the forms of the skeleton and dermal spi- 

 cula there is a close approximation to each other. The 

 most striking difference between the two species exists in 

 the characters of the retentive spicula. In H. Dickiei 

 the anchorate spicula are very much larger and greatly ex- 

 ceed in number those in //. incrustans, and in the former 

 there is a total absence of bihamate spicula, while in the 

 latter species they are very numerous and various in form, 

 so that these structural differences in addition to those of 

 external character will enable the student to readily distin- 

 guish the two species. 



The vast quantity and great size of many of the ancho- 

 rate spicula is a very remarkable feature in this sponge. 

 The inner surface of the dermal membrane is literally 

 crowded with them, attached to the membrane by the 

 middle of the convex surface of the curved shaft, while the 

 terminal flukes are thrown upward into the sarcode. They 

 are dispersed evenly over the whole surface of the mem- 

 brane without the slightest indication of order or arrange- 

 ment. They are very abundant also on the interstitial 

 membranes, but not to so great an extent as on the dermal 

 one. These spicula in the specimen under consideration 

 are found in all stages of development, from the young 

 and slender curved shaft, with scarcely an indication of the 

 terminal flukes, to the strong and fully developed spiculmn, 

 with the flukes nearly a third of its entire length. 



